1. Field of the Invention
This invention relates to a gas sampling system and more particularly to a gas sampling system for an ion mobility spectrometry (IMS) instrument that detects chemicals present as vapors in air or other gases, or liberated as vapors from condensed phases such as particles or solutions, when the source of vapors is at a distance from the entrance orifice of the IMS.
2. Description of Related Art
IMS instruments operate on the basis of the time taken by ionized molecules to move through a gas-filled drift region to a current collector while under the influence of an electric field. The ions are created in a gas-filled region called the ion source, which is connected to the drift region through an orifice or a barrier grid. The ion source may use any of a variety of techniques to ionize atoms and molecules. One or more flowing streams of gas enter the ion source through one or more orifices, and the gas may exit through one or more different orifices. At least one of the flowing gas streams entering the ion source includes gas that has been sampled (the “sample gas”) from the surrounding atmosphere or other source of vapor to be analyzed.
In some cases, the process of taking a sample begins with an operator rubbing an absorbent substance, such as chemical filter paper, onto the surface to be tested. Particles of the chemical of interest may then be transferred and concentrated on the absorber. This intermediate absorber may then be brought to the vicinity of the sampling orifice of the IMS. However, this method of concentrating using an absorbent substance is deficient in that it tends to be relatively slow to implement and is subject to variations in the skill of the operator. Additionally, while the absorber is relatively low in cost, the process of taking a great many samples becomes expensive in that the absorber generally should only be used once to ensure consistent results.
The instrument's sampling method uses a gas pump, which draws the sample gas into the ion source through a tube. For example, the pump may be disposed to provide a partial vacuum at the exit of the ion source. This partial vacuum may be transmitted through the confines of the ion source and appear at the entrance orifice of the ion source. A further tubulation may be provided as an extension to a more conveniently disposed sampling orifice external to the IMS. The operator may place a sample in the near vicinity of this external sampling orifice, and the ambient vapor may be drawn into the gas flow moving towards the ion source.
Sometimes molecules of interest undesirably adsorb onto surfaces in the sampling flow path. Therefore, it is sometimes useful to minimize unnecessary surfaces between the sampling orifice and the ion source. This is why, in some cases, the gas pump is often disposed in the gas flow stream following the ion source, rather than preceding the ion source.
The existing methods of gas sample collection are deficient in that it is difficult to adequately sample from a large volume unless the volume is a semi-sealed solid enclosure. Excessive dilution of the gas sample often occurs when the atmosphere is allowed to freely enter the space being sampled. Avoiding solid walls can be important in applications where wall surface contamination can be significant or the presence of nearby walls is unpleasant to the contained object or undesirable. The present invention particularly addresses the problem of large volumes, providing virtual walls to seal the sampled volume from the atmosphere.